1. Field of the Invention
The present invention relates to a die for producing co-extruded structural shapes, in which a core constituted by a mixture of polymeric materials, in particular of recycled polymeric materials, is provided with a polymeric coating (skin), said die comprising a central body with an internal duct destined to convey the molten material destined to constituting the core of the structural shape.
2. Description of the Prior Art
Dies used for producing co-extruded structural shapes constituted by a plurality of material, which may also be of different hardness and/or color, have been known for a long time in the sector. The operating way of said dies is based on the use of thermoplastic polymeric materials having well-defined rheological characteristics. When such characteristics vary, in particular with varying viscosity and average molecular weight values of the molten mass, the equilibria of correct advancement and distribution of the molten mass are compromised and the die results to be no longer suitable for the intended purpose.
Therefore, it is clear that the traditional technique used to design the co-extrusion dies cannot be transferred to the design of dies used to extrude mixtures of thermoplastic polymers derived from the differentiated collection (recycle) of wastes, from processing scraps, and so forth. The presence of various polymeric matrices chemically incompatible with one another leads to phase separations during molten material flowing through the extrusion head. In the attempts performed in the past and aiming at coating, by a co-extrusion technique, an extruded structural shape based on a heterogeneous mixture of thermoplastic polymers, with a protective and aesthetical skin based on a specific polymeric material, it was observed that, instead of uniformly distributing on the surface of an extruded structural shape, said skin material penetrates the boundary regions between different phases of the polymeric matrix, leading to articles of manufacture characterized by strong unevennesses in the distribution of the polymeric material which constitutes the sheltering skin.